Muffle construction

ABSTRACT

The wound wire heating element of an electrical resistance furnace muffle is enclosed within a conduit provided by a multiplicity of tubular refractory elements. Adjacent ends of the refractory elements are cooperatively configured to avoid gaps therebetween, and the refractory elements are dimensioned relative to the heating element to accommodate expansion thereof. In preferred embodiments, the heating element is fabricated from a platinum metal and the refractory elements are made of an alumina material that is substantially free from iron and its oxides.

United States Patent 1 1 Pilkington, Sr. et al.

1451 July 31, 1 973 1 MUFFLE CONSTRUCTION [75] Inventors: Russell T. Pilkington, Sr., Yucaipa; Russel T. Pilkiugton, Jr., Calimesa, both of Calif.

[73] Assignee: The J. M. Ney Company, Bloomfield,

Conn.

[22] Filed: Apr. 14, 1972 [21] Appl. No.:.244,020

52 us. c1 2 19/390, 13/22, 13/25,

219/406, 219/550, 338/213, 338/296 9 [51] Int. Cl. F2713 5/14 [58] Field of Search, 219/390, 406, 407,

[56] References Cited UNITED STATES PATENTS 596,160 12 1897 Helberger..... 338/213 i 554,910 2/1896 Delany 338/213 X 724,069 3/1903 Winter 219/406 1,096,414 5/1914 Coolidge... 219/424 1,234,499 7/1917 Smalley 219/390 1,930,836 10/1933 DAmico 219/390 2,963,529 12/1960 Schmidt 13/22 3,105,287 10/1963 Whearley et al 333/213 x FOREIGN PATENTS OR APPLICATIONS 501,009 2 1939 Gl'al 1311mm 13/22 343,358 8/1904 France 333/213 125,053 9/1942 Germany 13/22 Primary Examiner-Velodymyr Y. Mayewsky iAtt 0rney--Pete1' L. Costas I [57] ABSTRACT The wound wire heating element of an electrical resistance furnace muffle is enclosed within a conduit provided by a multiplicity of tubular refractory elements. Adjacent ends of the refractory elements are; cooperatively configured to avoid gaps therebetween, and the refractory elements are dimensioned relative to the heating element to accommodate expansion thereof. In preferred embodiments, the heating element is fabricated from a platinum metal and the refractory elements are made of an alumina material that is substantially free fromiron and its oxides.

ll C1aims,,7 Drawing' Figures 1 FIG-2 FIG.6

MUFFLE CONSTRUCTION BACKGROUND OF THE "INVENTION resistance heating element or as the result of deterioration in refractory parts. In such furnaces the resistance element will generally be supported in such a manner as to preventcontactlbetween adjacent turns,

of the wire, and .this has typicallybeen accomplished either by burying the wire in powdered charcoal or the like or by embedding it in a solidified refractory material. Although the solidified refractory affords better structural support and protection against contact and short-circuiting than does the particulate material, such embedment has also been found to be .quite unsatisfactory in a number of respects.

More particularly, notwithstanding considerable effort devoted to the development ofalloys and refractories having matched coefficients of thermal expansion so as to permit successful embedment, little successhas been realized. The high levels of stress which develop in the metal and refractory material as a result of different expansion rates cause .or. contribute fundamentally to cracking and ultimate fracture of the wire and/or the refractory body, rendering the furnace inoperative after relatively few thermal cycles. i

In addition, it is frequently desirable to utilize in such SUMMARY OF THE INVENTION It has now been found that the foregoing and related objects of the present invention can be readily attained in an electrical resistance furnace muffle comprising a wound elongated wire heating element and a multiplicity-.of tubular-refractory elements. The heating element provides a multiplicity of turns spaced apart along the length thereof and a firing chamber within the space enclosed thereby, and the refractory elements are disgPOSBd about the heating element and are in end-to-end I contact along the multiplicity of turns thereof; By mak I in; the passageway of the refractory elements greater f in cross sectionthan the heating element, spacing is provided thereabout to accommodate its expansion. In addition, adjacent ends of the refractory elements are cooperatively configured to mate and to avoid substanfurnaces heating elements fabricated from platinum or an alloy thereof because of the desirable heating characteristics afforded .and the high temperatures attain able therewith. However, platinum has been found to be prone to very rapid deterioration under the thermocycling conditions to which the heating elements are subjected in such a furnace, and it is believed that this is due to the susceptibility of that metal to attack by certain contaminants, especially iron and its oxides.

The problem tends tobe most serious in instances in which the heating element is embedded in the refractory, not onlybecause of the intimacy of contact, and therefore the ease of contamination which results, but also because replacement of the wire is virtually impossible. A

Accordingly, it is the primary object of the present invention to provide a novel electric furnace muffle affording an operating life of extended duration.

A more specific object is to provide such a muffle in which a wound resistance wire heating element is secu'rely supported about the firing chamber in such a manner as to minimize the possibility of contact between adjacent turns of the wire and consequent shortcircuiting. j

Another object is to provide such a mufi'le'in which the heating element is free to move relative to its supporting structure. I

Still another object is-to provide such a muffle inwhich improved protection against contamination is.

provided for the heating element thereof.

A further object is to provide a muffle having the foregoing features and advantages which is relatively simple and inexpensive to construct, and in which the resistance wire heating element may be readily replaced if necessary.

v fractory material tially any gaps 'therebetween, thereby providing .a substantially closed conduit for the turns of the heating 61- v ement. v

In preferred embodiments, the muffle is of polygonal cross section and the turns and assembled refractory elements conform thereto, with the refractory elements being substantially rectilinear. The most desirable polygonal cross sections is rectangular, and accordingly at least someof the ends of the refractory elements are desirably bevelled at about a 45 angle to cause adjacent ones thereof to extend at right angles to one another. The refractory elements maybe of polygonal external cross-sectionalconfiguration to permit stable stacking of one upon another, and in particularly preferred embodiments they are extruded of alumina. V

' In most instances, the muffle will additionally include a backing-structure extending about the refractory elements therewithin, and such a structure may be com I prised of a plurality of assembled refractory members which conform substantially to the external configuration of the refractory elements. Preferably, end portions of the resistance heating element extend through the backing structure and have refractory elements disposed thereabout to prevent contact with the backing structure. The particularly preferred embodiments utilize heating elements fabricated from a metal selected from the group consisting of substantially pure platinum, platinum alloys, and nickel/iron/chromiurn alloys, and when a platinum metal is employed the alumina reis desirably free from iron and iron oxides. I

BRIEF DESCRIPTION OF THE DRAWINGS the upper tube backing block partiallybroken away to expose the wire-enclosing tubular refractory elements thereof; H

FIG. 5 is an exploded perspective view of the assembly of FIGS. 3 and 4 with the upper backing block removed for clarity of illustration and drawn to a further enlarged scale;

FIG. 6 is a fragmentary sectional view along the line 6-6 of FIG. 3 drawn to a greatly enlarged scale and 3 with only some of the tubes and resistance wire illustrated; and

FIG. 7 is an exploded perspective view of a number of tubular refractory elements employed in th muffle assembly illustrated and drawn to the scale of FIG. 5.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT example, of aluminum) having a cylindrical sidewall 10 and a circular front wall 14. The front wall 14 has a rectangular opening 18 which affords access to the muffle assembly, generally designated by the numeral 12, contained therewithin. An outwardly extending drying shelf 16 is secured on the front wall 14 of the container below the opening l8.therein, and a removable door 20 is provided for engagement within the opening 18 when the furnace is in use. The door 20 will normally be fabricated from a refractory material, and it has a pair of recesses 22 formed therein to facilitate gripping for removal. The opposite end of the sidewall 10 is closed by a plate (not illustrated) made of asbestos or the like, which has extending outwardly therethrough a pair of plug prongs 24. The prongs 24 are connected to the ends of the resistance wire 26 (FIG. 5), enabling the muffle to be readily plugged and unplugged from a corresponding female receptacle within the furnace (not illustrated) with which the muffle is intended for use. The furnace will normally be pro vided with automatic temperature control means, and

the asbestos plate will therefore generally have an aper ture to receive the probe of a thermocouple heat sens ing device. The type of muffle illustrated is suitable for use with furnaces such as the MARK II and Model 650 modular vacuum furnaces commercially available from The J. M. Ney Company of Bloomfield, Conn.

With particular reference now to FIGS. 3 to 7, the resistance wire 26 of the muffle assembly 12 is disposed as a generally rectangular coil providing the heating element therein. A multiplicity of tubular elements 28 are mounted along substantially the entire length of the wire 26, and the elements 28, which may be extruded from high purity alumina refractory material, have a generally rectangular external cross section. The adjacent ends 30 of the elements 28 are bevelled at an angle of approximately 45 to dispose the elements 28 at right angles to one another and to thereby define the rectangular path for the wire 26. The tublar elements 28 are supported within a backing structure or frame provided by assembled and cemented upper and lower support blocks 32 and end blocks 34 which are also fabricated from an appropriate refractory material, and the elements 28 may be bonded to the blocks 32, 34 if so desired. The end blocks 34 are provided with notched corners 36 which are dimensioned to receive the outer ends of the elongated tubular elements 28, which are.

an appropriate refractory cement to provide a selfsupporting assembly.

As is best illustrated in FIG. 6, the passageway 38 of each of the tubular elements 28 is sufficiently large to receive the wire 26 with spacing therebetween, and to thus permit free relative movement so as to avoid undue stress and the resultant cracking or deterioration of the wire 26 and/or of the elements 28. FIG. 5 best illustrates the close contact that is achieved beween adjacent elements 28 andthe consequent absence of spaces or gaps between the bevelled ends 30 thereof. The closed conduit of high purity alumina' which is thereby defined. and in which'substantially theentire length of the wire heating element 26 is enclosed very effectively protects the wire 26 from contamination and thereby appreciably extends the operating life of the unit. The closely and securely supported elements 28 also ensure the substantial absence of gaps through which the wire 26 might expand, and thereby prevent contact or arcing between adjacent turns which would cause short-circuiting and premature burnout.

Although the alumina utilized for the tubular elements 28 may have an aluminum oxide content in the range of about 85.0 to 99.9 percent by weight, preferably the amount of aluminum oxide will exceed about 99.0 percent by weight of the material. When the heating element is fabricated of platinum or an alloy thereof, the purity of the alumina tubes is believed to' be especially significant. In particular, it is believed that an appreciable content of iron (which may be naturally present in oxide form or may result from extrusion or grinding operations) tends to contaminate the wire and to thereby cause deterioration and rapid burnout. The

density of the alumina is also significantand it'should be as high as possible for best results. Not only'is density important from a structural standpoint, but it is also fundamental to the efficiency of the muffle because low density or'porous elements tend to have a heat insulating, rather than conducting, effect.

As will be appreciated, the wall thickness of the tubu- 4 and attainment of desired temperatures during heating metal container of the muffle by cementing peripheral cycles. Accordingly, the tubular elements will generally have walls ranging from about 10 to 40 mils in thickness; although values at the lower end of the range'are preferred from the standpoint of heat transfer and may be of adequate structural strength under certain conditions, the practical lower limit is presently about 20 mils due to extrusion limitations. I

The rectangular external configuration of thetubular elements facilitates their stable stacking upon one an-v other and provides flat surfaces within the muffle aboutthe firing chamber. However, other cross sectional configurations (e.g., round, hexagonal, triangular, etc.) may certainly be used, and may offer the advantage of increased surface area for radiation into the firing,

chamber. The rectilinear configuratio'n of the tubular elements is desirable from the standpoint of ease of extrusion and end joint formation, but curvilinear config urations are also feasible and may be preferred to achieve certain dispositions of the heating element. The configuration of the ends of the tubular elements may also vary, and in some cases will be dictated by the changes in direction that are to occur between adjacent elements. The bevelled surfaces illustrated are desirable from the standpoint of simplicity and ease of production, but other mating or interengaging configurations may be employed, such as may be formed by rabbetting in any convenient manner to cooperatively provide substantially closed joints.

As mentioned previously, the wire heating element is desirably fabricated from substantially pure platinum metal or an alloy thereof, such as a combinationof platinum with up to .13 percent by weight of .rheniurn. Alternatively, and. notwithstanding the protection provided by the tubular elements employed, it may in some cases be preferred to utilize a metal for the heating element that is less vulnerable to deteriorative contamination and is less expensive. Any of the commercially available base metal heating alloys may be employed for that purpose, typically of which are the nickel/- chrominum alloys (which normally contain iron, and other elements in some instances); a suitable commercial alloy of this type containing nickel, iron, chromium and carbon (about 60,24, 16 and 0-1 percent, respectively) is available-under the trademark NICHRQME. Such alloys are often capable of attaining the temperatures of about 2250 Centrigrade, although the actual temperature developed will of course depend upon the current provided to the heating element and its diameter.

Exemplary of the efficacy of the present invention are the following specific examples:

EXAMPLE ONE A muffle having substantially the construction illustrated is assembled utilizing a pure platinum wire heating element and commercially available extruded tubes of alumina refractory material. The wire has a diameter of about 0.015 inch, and the tubes have an inside bore of about 0.030 inch and a square outside configuration measuring about one-eighth inch on a side; the surfaces of the tubes arecontaminated with small particles of iron. The muffleis installed in a test furnace and repeatedly cycled between temperatures of about 650 to l3l5 Centigrade. Failure occurs after approximately 300 such cycles, and the efficiency of operation in terms of response and heat transfer is found to be poor. Nevertheless, the ceramic elements are found to be in good condition at the end of the test, and the heating element can readily be replaced for renewed operation.

EXAMPLE TWO A muffle similar to that of Example One is constructed, utilizing platinum wire of about 0.025 inch diameter and tubes of 99.5 to 99.9 percent alumina having a square external configuration measuring about 0.15 inch on a side, a wall thickness of about 0.02 inch (providing a square passageway), and a length of about 3 inches. After installation in a suitable furnace, the muffle is subjected to thermal cycling as described in the previous Example. In contrast to the results thereof, it is found that the muffle of this Example can safely be heated from about 650 to l3l5 Centigrade in about 1.75 minutes, and a highly desirable heating profile is exhibited. Moreover, the muffle can be heated through approximately 2500 cycles before any failure occurs.

Thus, it can be seen that the present invention provides a novel electrical resistance furnace muffle affording an operating life of extended duration. More specifically, it provides a mufile in which a wound resistance heating element is securely supported about the firing chamber in such a manner as to minimize the possibility of contact and short-circuiting between ad- .jacent turns thereof. The heating-element is free to move relative to the supporting structure of the muffle, and improved protection against contamination is provided therefor. The muffle is relatively simple and inexpensive to construct, and the wire heating element may be readily replaced, if necessary. I

1 Having thus described the invention, we claim:

.1. In an electrical resistance furnace muffle,'the'com f" A I I bination comprising:

a. a wound elongated high temperature resistance wire heating element extending generally in a spiral and providing a multiplicity of turns spaced apart along the length thereof and defining a firing chamber of generally polygonal cross section within the space enclosed thereby; and

b. a multiplicity of substantially rectilinear tubular refractory-elements disposed about said heating element in end-to-end contact along said multiplicity of turns, said refractory elements being assembled in side by side contact to provide walls of said firing chamber and defining a polygonal cross section for said muffle and said firing chamber, the passageway of said refractory elements being of greater cross section than the cross section of said heating element to provide spacing thereabout, the adjacent ends of said refractory elements being open and cooperatively configured to mate and avoid substantially any gaps therebetween, said refractory elements providing a substantially closed conduit for said turns of said heating element and accommodating expansion thereof.

2. The muffle of claim 1 wherein said polygonal cross section is rectangular and at least some of said ends of said refractory elements are bevelled at about a 45 angle to cause adjacent refractory elements toextend at right angles to one another.

3. The muffle of claiml wherein said refractory elements are of polygonal external cross-sectional configuration to permit stable stacking of one upon another.

4. The muffle of claim 1 additionally including a backing structure extending about said refractory elements.

5. The muffle of claim 4 wherein theend portions of said resistance heating element extend through said backing structure and wherein refractory elements are disposed about said end portions to prevent contact thereof with said backing structure.

6. The muffle of claim 1 wherein said refractory elements are extruded alumina and wherein said heating elementis fabricated from a metal selected from the group consisting of a substantially pure platinum, plati substantially pure platinum, platinum alloys and nickel/iron/chromium alloys, and providing a multiplicity of turns spaced apart along the length thereof and defining a firing chamber of rectangular cross section within the space enclosed thereby;

b. a multiplicity of substantially rectilinear tubular refractory elements disposed about said heating element in end-to-end contact along said multiplicity of turns, said refractory elements being assembled in side by side contact to provide walls of said firing chamber and defining a rectangular cross section larly extending conduit for said turns of said heating element and accommodating expansion thereof; and

c. a backing structure extending about said refractory elements, said backing structure being comprised of a plurality of assembled refractory members conforming substantially to the external configura- 4 tion of said refractory elements.

9. The muffle of claim 8 wherein said refractory elements are extruded alumina.

10. The muffle of claim 9 wherein said heating element metal is selected from the class consisting of sub- 1 stantially pure platinum and platinum alloys, and J wherein said alumina refractory material is substantially free from iron and iron oxides.

11. The muffle of claim 10 wherein the end portions of said heating element extend through said backing structure, wherein refractory elements are disposed about said end portions to prevent contact thereof with said backing structure, and wherein said adjacent ends of said refractory elements are bevelled at an angle of about 45.

men al: 

2. The muffle of claim 1 wherein said polygonal cross section is rectangular and at least some of said ends of said refractory elements are bevelled at about a 45* angle to cause adjacent refractory elements to extend at right angles to one another.
 3. The muffle of claim 1 wherein said refractory elements are of polygonal external cross-sectional configuration to permit stable stacking of one upon another.
 4. The muffle of claim 1 additionally including a backing structure extending about said refractory elements.
 5. The muffle of claim 4 wherein the end portions of said resistance heating element extend through said backing structure and wherein refractory elements are disposed about said end portions to prevent contact thereof with said backing structure.
 6. The muffle of claim 1 wherein said refractory elements are extruded alumina and wherein said heating element is fabricated from a metal selected from the group consisting of a substantially pure platinum, platinum alloys and nickel/iron/chromium alloys.
 7. The muffle of claim 6 wherein said heating element metal is selected from the class consisting of substantially pure platinum and platinum alloys, and wherein said alumina refractory material is substantially free from iron and iron oxides.
 8. In an electrical resistance furnace muffle, the combination comprising: a. a rectangularly wound, elongated, high temperature resistance wire heating element fabricated from a metal selected from the group consisting of substantially pure platinum, platinum alloys and nickel/iron/chromium alloys, and providing a multiplicity of turns spaced apart along the length thereof and defining a firing chamber of rectangular cross section within the space enclosed thereby; b. a multiplicity of substantially rectilinear tubular refractory elements disposed about said heating element in end-to-end contact along said multiplicity of turns, said refractory elements being assembled in side by side contact to provide walls of said firing chamber and defining a rectangular cross section for said muffle and said firing chamber, the passageway of said refractory elements being of greater cross section than the cross section of said heating element to provide spacing thereabout and the external cross sectional configuration of said refractory elements being polygonal to permit stable stacking of one upon another, at least some of the adjacent ends of said refractory elements being open and cooperatively configured to mate and avoid substantially any gaps therebetween and to cause adjacent refractory elements to extend at right angles to one another, said refractory elements providing a substantially closed rectangularly extending conduit for said turns of said heating element and accommodating expansion thereof; and c. a backing structure extending about said refractory elements, said backing structure being comprised of a plurality of assembled refractory members conforming substantially to the external configuration of said refractory elements.
 9. The muffle of claim 8 wherein said refractory elements are extruded alumina.
 10. The muffle of claim 9 wherein said heating element metal is selected from the class consisting of substantially pure platinum and platinum alloys, and wherein said alumina refractory material is substantially free from iron and iron oxides.
 11. The muffle of claim 10 wherein the end portions of said heating element extend through said backing structure, wherein refractory elements are disposed about said end portions to prevent contact thereof with said backing structure, and wherein said adjacent ends of said refractory elements are bevelled at an angle of about 45*. 